There are generally two types of liquid delivery tubes used on board an aircraft or other aerospace vehicle—vacuum waste tubes and tubes used to carry potable water from a potable water tank to a hand washing station, sink, or other water-using apparatus. Both types of water tubes are typically made out of corrosion resistant steel (CRES) thin walled tubing. For example, current vacuum waste tubes are typically titanium thin walled (0.020″ to 0.028″) tubes of diameters from one to four inches in diameter. In some situations, corrosion resistant steel (CRES) thin walled tubing, which is about 0.020″ to 0.035″ in wall thickness, is used. These tubes are used because these metals meet all aerospace requirements for transport elements (temperature, chemical exposure, structural, impact, and other requirements). Tubes used for the vacuum waste system are primarily straight tubes, which also incorporate bends and wyes (manifolds, pullouts, tees, and so forth). FIG. 1 shows a waste tube which has a wye (pullout) and various bends. Typically, a straight wall titanium tube is bent as required and wyes are welded and fittings (AS1650 style) are swaged or welded to tube ends. In some cases, a beaded end is used per AS5131 in place of the welded fittings.
In the event that a hard object (such as a battery, a cell phone, or other flushable object that is not intended to traverse a vacuum sewer line) is flushed into the vacuum waste system, becoming a projectile, the impact at bends or wyes could break the tube and lead to system failure. Titanium waste tubes are generally used because they are lightweight and handle impact requirements and the vacuum pressure (typically 0 to −11 PSID) cycling of the vacuum waste system. CRES tubes also meet this requirement and while they are less costly, the weight of CRES increases over titanium. CRES (which has a density of about 0.29 lbs/in3) is approximately 60% heavier than titanium (which has a density of about 0.163 lbs/in3).
The other types of water tubes on an aircraft, potable water tubes (e.g., the tubes that for transporting potable water throughout the aircraft), are typically CRES thin walled (0.020″ to 0.035″) tubes of diameters from about a half inch to about 5 inches in diameter. Titanium may also be used when a lightweight system is required and higher cost is feasible. For areas where complex routing (bends) is required, flexible hoses (for example, AS4468, AS5420 or similar) are used. Water tubes do not have an internal impact requirement but must meet potability requirements (NSF/ANSI Standard 61 or equivalent) and have pressure requirements of 125 PSID proof and 188 PSID burst.
Potable water tubes used are primarily straights, wye (pullout, manifold, tee) and bends. FIG. 2 shows a typical water tube straight with a pullout. Typically, if a straight tube needs to be bent, wyes are welded and fittings (AS1650 style) are brazed or welded to tube ends. Since tube diameters are relatively small, CRES is used in place of titanium for cost savings. Titanium would decrease weight but increase cost.
However, it is desirable to provide waste and water tubes of other materials that are lightweight, that meet the required strength and impact requirements, and that can be manufactured in the desired configurations. In some instances, it is desirable to manufacture tubes with varying diameters, varying lengths, shapes, and curvatures. For example, because the aircraft or other passenger transport vehicle may demand a tortuous waste or water route, the tubes should be designed in such a way that they can have bends or turns easily formed therein. It is also desirable to reduce costs of the tubing, such that their manufacture does not require complicated and expensive tooling in order to manufacture the tubing.